I woke up this morning and the first conscious thought I had was, "I have to get that broken stub out." I removed the new water pump (the broken bolt had, of course, been the last one in), and got my little Craftsman extractor kit and my drill. (I would like to mention my Makita cordless drill here - i've had it for 12+ years, abused it heavily, and it's been great.) I started drilling with the reverse bit and to my astonishment the stub spun right out - it wasn't even tight. It makes sense now, of course, because the bolt doesn't tighten into the threaded socket, it tightens the pump against the block. Anyway, I was pretty happy, although I have to find a new bolt now.